The present invention relates in general to a serial impact printer and to a novel print element therefor. In particular, the present invention relates to a serial impact printer and a type element useful therewith for the formation of chemical formulae interspersed with ordinary alphanumeric characters on the same page or document.
The typing of chemical formulae has generally been a rather tedious job. Early prior art general purpose typewriters made no provision for the typing of such formulae. Accordingly, when typing such formulae, the typist was required to insert certain subscripts, superscripts, arrows and symbols by hand, for example with pen and ink. With the development of replaceable print elements, such as typing balls and daisy type printwheels, the aforementioned problem was somewhat alleviated since a general purpose typewriter designed primarily to form alphanumeric characters could be modified by changing print elements to form alphanumerics as well as some chemical symbols, thus allowing the typing of text interspersed with chemical formulae on the same page.
The aforementioned replaceable print elements are limited, however, as to the number and diversity of chemical symbols which can be formed since only a finite number of characters can be placed on a single print element, if at least an upper and lower case alphanumeric character set is to be also provided such that text and chemical symbols may both be formed with the same element. This problem is severe when daisy type printing elements are utilized. In daisy wheel type printing elements, the number of characters is limited by the number of daisy wheel spokes and the number of spokes is limited by the fact that each character requires a minimum size for legibility and therefore the distance between the distal ends of wheel spokes may not be decreased without limit. Further, the distance between distal ends of the spokes may not be increased by substantially increasing the wheel diameter since increasing the diameter increases inertia of the daisy wheel and thus decreases the printing speed. Thus, the provision of a printing element useful in the formation of alphanumeric characters as well as chemical symbols requires some compromise in the ability to form all the characters or symbols which might be desired. In Vydec Chemical Option Reference Manual, 4093--0020, copyright 1979 by Vydec Engineering Publications, a daisy type print element is disclosed having a chemical character set. While the Vydec print element permits the typing of alphanumeric characters interspersed with chemical elements on the same page of text, the Vydec printwheel suffers from certain disadvantages.
First, the Vydec character set employs only a full size number set. Since many chemical formulae require relatively small subscripts and superscripts, as well as full size numerals, it would be desirable to provide a print element for forming alpha-numeric as well as symbols with two full numeral sets, one being full size and the other being half size (hereinafter referred to as upper and lower case numerals respectively) without substantially increasing the number of daisy printing element spokes.
Another disadvantage of the Vydec print wheel is the inability to form closed ring structures of the benzene type of varying size with that wheel because of the inability to form continuous chemical bonds of varying length. It is an object of the present invention to provide a printing element for use in the formation of text as well as chemical formulae which is useful in forming closed ring-like structures and other like structures having varying sizes.
Still another problem associated with prior art impact printers employing chemical printing elements is the requirement that the typist shift the position of the typewriter platen with respect to the print point each time a subscript or superscript is formed. Since the number of subscripts and superscripts formed when typing chemical formulae may be large, the necessity for the typist to shift the relative position of the print element with respect to the platen is tedious thus decreasing typing speed. It would be desirable to provide a serial impact printer with the capability of automatically shifting the relative positions of the print point and platen automatically when subscripts or superscripts are formed.